It is commonly known that fluorescent colors provide increased conspicuity for visual signaling under most lighting conditions, and particularly under low natural lighting conditions. These conditions occur at dusk and at sunrise and present a challenge for traffic sign manufacturers. If increased conspicuity of an article is desired, the article is often colored with fluorescent colorants. Fluorescent colors allow enhanced conspicuity because the visual contrast that the fluorescent colors create with the environment is greater than that of similar non-fluorescent articles. Fluorescent colored traffic signs are effective at increasing the conspicuity of the signs and, in turn, increase the likelihood of the signs being observed.
Certain conventions have evolved with respect to the color of traffic signs. For example, informational signs may be green or blue, caution or warning signs may be orange or yellow, and yield and stop signs are red. Motorists have become accustomed to these conventions and color purity is important to encourage the appropriate response. Although red fluorescent dyes are available, their chromaticity or color purity can vary from pink to orange in dyed articles and may not have a true red appearance.
In addition to color purity, it is important for highway signage to be visually conspicuous. As the brightness or luminance of a sign increases, the more visually obvious the sign is relative to its background. The luminance of a fluorescing article is the sum of its reflected and emitted light. The greater the contribution from emitted light to total luminescence the brighter the appearance of the fluorescing article. It is therefore desirable to avoid conditions that quench fluorescence.
Various attempts to improve the visibility of signage have been reported. Burns et al., U.S. Pat. Nos. 5,920,429, 5,754,337, and 5,674,622 disclose fluorescent yellow articles comprising two fluorescent dyes dispersed in a polymeric matrix suitable for applying to retroreflective surfaces. The resulting yellow retroreflective articles are suitable for traffic signs.
Nagaoka et al., U.S. Pat. No. 6,048,069 disclose retroreflective sheeting having image elements made from light transmitting prismatic sheeting. An adhesive layer adheres the prismatic sheeting to the retroreflective sheeting and the adhesive may contain a red fluorescent dye.
Sakaya et al., European Patent Application No. 792,907 disclose polyolefin resin films suitable for outdoor use. The films contain fluorescent substances including perylene dyes and a shielding agent to suppress the deactivation of the fluorescent substance under outdoor conditions and from other substances in the film.
Nagaoka, International Application No. PCT/US97/17457 discloses a retroreflective graphic article having an image layer disposed on a retroreflective base sheet and a dye receiving layer disposed on the image layer. The image layer contains a pigment and a fluorescent dye. The fluorescent dye migrates to the receiving layer, thereby reducing adverse effects from the pigment on the visual properties of the dye.
Smith, International Application No. PCT/AU96/00303 discloses a method for making fluorescent dye compositions useful for making luminescent sunlight collectors from a solution of a fluorescent dye and a polymerizable monomer wherein the solution is prepared without additional solvent and with the aid of ultrasonic mixing. The monomer solution is polymerized in a vertical tube to form a luminescent sunlight collector.